1. Field of the Invention
The present invention relates to a roll stamper for continuously producing substrates for information recording media by forming preformats on a resin sheet. It also relates to a process for producing the roll stamper.
2. Related Background Art
Preformats such as track grooves or address pits are formed on substrates for information recording media such as optical disks. As a method of continuously mass-producing such substrates, Japanese Patent Application Laid-open No. 56-86721 discloses a method in which a thermoplastic synthetic resin sheet is heated and softened using an infrared lamp and thereafter signals in the form of grooves or pits are transferred using a molding roll.
The roll stamper used in such a method is prepared by sticking a flat-plate thin stamper on a mirror-finished roll substrate with an adhesive or the like or mechanically fastening the stamper on the substrate by means of a jig or the like, or by directly forming a preformat pattern on the body of a roll substrate.
As the preformat pattern formed on the roll stamper is used a preformat pattern similar to those used in conventional methods in which the substrate is formed sheet by sheet as in the injection process, the compression process and the photopolymerization process (hereinafter "2P process"), which pattern is substantially circular, for example, in the case of a disk.
With regard to the accuracy of the preformat pattern provided on the stamper used in the conventional injection process, compression process or a 2P process, tracking errors tend to occur unless a deviation from a circle is controlled to be not more than 100 .mu.m in the case of the disk.
When preformats are, however, continuously formed on a resin sheet, a relatively large pressure is applied to the roll stamper or shrinkage may occur with the cooling of resin, in a direction perpendicular to the direction in which the resin sheet is transported. Hence, in the case where the roll stamper on which the above substantially circular pattern has been formed is used, there is the problem that the preformat transferred onto the resin sheet causes a lowering of transfer accuracy, specifically, a lowering of the circularity or roundness. In particular, in the case of recording media like optical disks on which fine patterns are formed and information with a high density is recorded and reproduced, the lowering of the roundness may cause errors such as tracking-off.
This is also a problem in the case of molding a substrate for an optical card by a roll stamper, particularly resulting in a problem that the optical card substrate will become a defective product out of the specification due to the change in track pitch of the preformat pattern by shrinkage of the resin sheet, and it has been desired to have a countermeasure against the deformation of the preformat pattern accompanied with shrinkage of the resin sheet, particularly shrinkage in a direction vertical with respect to the conveying direction of the sheet.